U.S. Pat. No. 5,036,341 discloses an electrographical printer which, compared to electrophotographical or electrographical procedures, includes constructional simplifications and manufacturing cost savings. By using an electrode means, which is controlled according to the configuration of the desired pattern, certain openings of the electrode means are electrostatically opened and closed by an electrostatic field produced directly between the particle carrier and a paper sheet to achieve the technical simplification mentioned above. However, until now, a difficult problem to solve has been that the toner particles moving from the particle carrier through the electrode means are adhered to the carrier and form toner agglomerate, which successively causes deterioration of the printing quality and sharpness. The electrode means may include electrodes situated in rows extending in two transverse directions so that the rows of electrodes cross each other and conductors provided on an insulating carrier having pervasive apertures or the like.
A method described in European Patent No. 476,041 aims to correct this problem by blowing away the stuck toner particles using compressed air. However, only a very limited overpressure can be used because the toner particles must not blow out over the entire apparatus. Therefore, only a limited effect is achieved by this method.
Another problem which affects the transcription quality is the existence of variations in the relative distance between the electrode means and the particle carrier, between the electrode means and the back electrode, and between each of the electrode means, the particle carrier and the back electrode and the passing paper sheet. These variations depend on irregularity of the electrode means, which must be more accurately and precisely formed because the distance between the particle carrier and the back electrode is only about one or a few tenths of a millimeter.
Another problem is that the electrostatic field that is produced is disturbed by the presence of the toner particles and the presence of charges, for example ionized gases and recharged molecules which have been recharged by friction, on the insulating layer of the electrode means. Also, the insulating layer on the electrode means is charged by the toner particles.
These problems cause the printing quality to deteriorate after several pages if no actions are taken for cleaning the electrode means or charge control device.
European Patent 476,041 also teaches using a rotating magnet core in a developer roller to retrieve the toner particles from the electrode means, which is achieved by means of an extra strong cleaning magnet. The method is efficient, but has drawbacks, such as the attraction of thick layers of toner away from the toner container. The system requires that the rotation direction of the developer roller be reversed to clean the roller by means of a cleaning blade turning towards and away from the roller. The cleaning magnet can later be rotated to clean the electrode means. Tests have shown that the method is useful, but problems can occur because of the poor reliability of the cleaning blade and the magnetic influence of the cleaning magnet on the toner layer during the printing process.
Also, it is not enough to clean and remove the toner particles from the electrodes. A stable charge level must be maintained on the insulating layer of the electrodes. A method for maintaining a stable charge level is described in U.S. Pat. No. 5,307,092. According to this patent, a semiconducting surface material is provided on the electrodes to lead away the charges from the electrode or an antistatic layer, which is intermittent (for example, through continuous connection to ground), to lead away tribo charges.